What is different with retroviruses in reproduction is that they do not kill their hosts at first because they can still insert their own genome. This process is what is known the reverse transcription. This is done by different proteins.
I think that since it lives on the ocean floor, at the deepest of the ocean. So anyway, the fin on the Eurypterids is still necessary but not that much, since they live on the ocean floor. Their movements mostly on the ocean floor. So fin would not help them "stick" to the floor. Also there are still gravity acts on the Eurypterids and the feet would help them move faster. Hope this helps.
The correct answer would in fact be C
To prevent cancer, over exposure to radiation
